We are studying the developmental and phylogenetic aspects of lymphocyte diversity in a reptile, Chelydra. Our approach is to detect antigenic differences between lymphocyte subpopulations and to correlate these with functional aspects, such as responsiveness to mitogens and cellular kinetics of antibody production. A detailed study of the histogenesis of lymphoid organs and cells will serve as a baseline for analysis of the development of cell diversity and immunocompetence. A separate study has determined that the nutritional state of turtles acutely affects their immune status. A mild form of malnutrition results in acute thymic involution and immunodeficiency. This condition, however, appears to be reversible. Further studies are being conducted to assess the effects of more severe malnutrition and the extent of its reversibility with improved diet. A good knowledge of husbandry is essential in comparative studies on the immune system of uncommon animal species. Lastly, we are studying various aspects of the structure and function of the chicken bursa. These include the ultrastructural analysis of lympho-epithelial relationships, the effect of testosterone on various bursal cell populations and the mechanisms of antigen uptake by the bursa via the intrabursal and intravenous routes.